“It’s very troubling,” Nolan said. “He’s our attorney, and he’s supposed to represent our interests, but at the same he’s taken a very public stance against the Central Subway.”

At the agency’s board meeting Tuesday, Nolan plans to ask his colleagues to join him in directing SFMTA Executive Director Ed Reiskin to seek a state bar investigation.

If officials see a conflict, Nolan said, he’ll recommend that his agency hire outside counsel for matters related to the subway.

Although the mayoral election is less than a month away, Nolan said Herrera could be involved with the Central Subway project “for years” if he is not elected mayor and remains city attorney.

“This is nothing against him personally, but he has gone out of the way to trash this project,” Nolan said. “That leaves me wondering how open-minded and objective he can be.”

Herrera’s campaign spokesman Matt Dorsey said just because the city attorney doesn’t agree with the subway plan doesn’t mean he can’t defend it.

“A city attorney’s role is to advise on the legal aspects of city policies, not the wisdom of them,” said Dorsey. “All elected officials fulfill public duties they sometimes disagree with, but they still have to do their jobs. That’s particularly true for attorneys — lawyers who defend bank robbers aren’t required to believe that robberies are a good idea. It’s about advising on the legal aspects of the issue, not the wisdom.”